Field of the Invention
The present invention relates to a spark-ignition engine and, more particularly, to a multi-point spark-ignition engine which has a combustion chamber provided with a plurality of cavities for holding a combustible air-fuel mixture and which further includes spark plugs disposed near the centers of the cavities to stably and rapidly burn the mixture even if it is lean.
Description of the Prior Art
It has been impossible for the prior art engine to achieve combustion of lean air-fuel mixture simply by making the supplied air-fuel mixture leaner, because combustion of the lean mixture reduces the burning velocity and renders the combustion unstable. Accordingly, various attempts have been made to stabilize the combustion. For example, the fluidity of air-fuel mixture, or the squish and swirl effects, was utilized. The distances propagated by flames were shortened by multi-point ignition (Japanese Patent Laid-Open No. 132306/1976). The compression ratio was increased to shorten the ignition delay (SAE 790386).
However, none of these attempts permitted stable operation with quite lean air-fuel mixture that reduces the NO.sub.x emission contained in the exhaust gas to a sufficiently low level. Therefore, EGR (exhaust-gas recirculation), ignition delay, or other countermeasure is needed. This has frequently made it impossible to make the best use of the features of combustion of lean mixture.
In an internal combustion engine, the theoretical thermal efficiency is given by EQU .eta..sub.th =1-(1/.epsilon.).sup.k-l ( 1)
where .epsilon. is the compression ratio, and k is the ratio of specific heats, Cp/Cv. (Cp and Cv are the specific heats at constant pressure and constant volume respectively). It can be understood from this equation that the theoretical thermal efficiency is improved by increasing the compression ratio or the ratio of specific heat. That is, the best result is obtained by increasing the compression ratio and allowing combustion of the lean mixture. It is to be noted that as the air-fuel mixture approaches air, the value of the ratio of specific heat k increases. Further, combustion of the lean mixture improves the antiknock property and reduces the temperature of the combustion gas, leading to reductions in heat loss and in the amount of NO.sub.x emission. These features have certain attractions, and this system has been put into practical use in present vehicles. However, this system is unable to run with such a lean mixture that can sufficiently suppress the level of the NO.sub.x emission in the whole range.
A so-called stratified combustion svstem differing from the premix lean combustion has been proposed. Specifically, a relatively rich air-fuel mixture that is relatively easy to ignite is supplied only in the vicinities of ignition sparks, but the inside of the cylinder is generally supplied a with a lean air-fuel mixture. Since the mixture further approaches air, a higher thermal efficiency can be attained. Further, a sufficient amount of air is supplied, greatly reducing poisonous components of the exhaust gas.
However, this stratified combustion system also has various difficulties. Specifically, it is difficult to control the air-fuel mixture. A high energy ignition svstem is necessitated. Instability in the combustion increases the THC emission in the range of low loads.